Closed loop servo systems have long been used to rapidly position magnetic read/write heads, print wheels and carriages and other devices. One such servo system is described in U.S. Pat. No. 3,663,880 wherein a difference count establishes how many data tracks a read/write head must move past to reach a desired data track. The difference count is decremented by one count as the head moves past each data track, with the difference count eventually being decremented to zero. Prior to the time that the difference count is decremented to zero, the motor providing the drive for the read/write head is energized by a signal indicative of the instantaneous velocity of the motor shaft providing support for the head.
In systems like the patented system, a critical phase of operation is when the difference count has decremented to zero and there is still a fractional count left. The normal procedure, as described in the referenced patent, is to then switch to position mode operation wherein the motor drive is energized with a signal indicative of the position of the motor shaft providing support for the head. However, even when the difference count has decremented to zero, there is a positional error as well as a velocity error. Those errors put a transient into the system and the head does not settle fast and clean, that is, the head toggles up and back about the desired track before coming to rest.